1. Field of the Invention
This invention generally relates to binary non-return to zero (NRZ) communications and, more particularly, to a system and method for determining the mean NRZ data stream rate. Additionally, applications of the NRZ data stream rate determination process are presented for acquiring voltage controlled oscillator (VCO) frequency ranges.
2. Description of the Related Art
Voltage controlled ring oscillators are commonly used in monolithic clock data recovery (CDR) units, as they""re easy to fabricate and provide reliable results. Ring oscillators obtain their tuning characteristics by changing the variable delay around the ring, usually in response to a dedicated control voltage input (tuning voltage). Voltage controlled ring oscillators can, and usually do exhibit a tuning range much wider than the closed loop phase-locked loop (PLL) bandwidth of the circuits in which they operate.
Clock recovery PLLs generally don""t use phase-frequency detectors (PFDs) in the data path since the incoming data signal isn""t deterministic. PFDs are more typically used in frequency synthesizers with periodic (deterministic) signals. Clock recovery PLLs use exclusive-OR (XOR) based phase detectors to maintain quadrature phase alignment between the incoming data pattern and the re-timed pattern. XOR based phase detectors have limited frequency discrimination capability, generally restricting frequency offsets to less than the closed loop PLL bandwidth. This characteristic, coupled with the wide tuning range of the voltage controlled oscillator (VCO), requires CDR circuits to depend upon an auxiliary frequency acquisition system.
There are two basic PLL frequency acquisition techniques. The first is a VCO sweep method. During an out-of-lock condition, auxiliary circuits cause the VCO frequency to slowly sweep across its tuning range in search of an input signal. The sweeping action is halted when a zero-beat note is detected, causing the PLL to lock to the input signal. The VCO sweep method is generally used in microwave frequency synthesis applications. The second type of acquisition aid, commonly found in clock recovery circuits, uses a PFD in combination with an XOR phase detector. When the PLL isn""t locked to a data stream, the PLL switches over to a PFD that is driven by a stable reference clock source. The reference clock frequency is approximately equal to the data stream rate. Thus, the VCO frequency is held very close to the data rate. Keeping the VCO frequency in the proper range of operation facilitates acquisition of the serial data and maintains a stable downstream clock when serial data isn""t present at the CDR input. When serial data is applied to the CDR, the XOR based phase detector replaces the PFD, and data re-timing resumes.
It would be advantageous if the mean data rate of an NRZ data stream could be simply measured.
It would be advantageous if a CDR or a clock synthesis unit (CSU) had the ability to operate over a broad range of clock frequencies using simple data rate analysis circuitry.
The present invention automatic data stream rate measuring system can be used as an acquisition aid for phase-locked loops in clock recovery applications. The system examines transitions in the data stream, counting those events in a given time frame, or logging the time required to accumulate a fixed count. Whether it be time or event counting, the results can be decoded into frequency band information pulling the VCO frequency into the correct range of operation, establishing a reference clock frequency for support during serial data outages, and enabling clock recovery action on the data stream.
Accordingly, a system is provided for measuring pseudorandom non-return to zero (NRZ) data rates in a communications device integrated circuit (IC). The system comprises a transition detector to sample a pseudorandom NRZ data. stream and to supply a mean frequency of transitions (Fd). The system further comprises a probability analyzer to receive the mean frequency of transitions, compare the mean frequency of transitions to a transition probability (P), and supply a derived mean data stream rate. The probability analyzer supplies the derived mean data stream rate (B) as follows:
B=Fd/P.
The transition detector can be set to supply a mean frequency of transitions (Fd) in response to one of the following: positive transitions having a 0.25 probability of occurrence; negative transitions having a 0.25 probability of occurrence; and, both positive and negative transitions having a 0.5 probability of occurrence. The transition detector samples a pseudorandom NRZ data stream includes sampling n data bits, and the probability analyzer derives the mean data stream rate with a standard deviation as follows:
"sgr"=SQRT((P)(1xe2x88x92P)(n)).
In some aspects of the system, a gating circuit is included to supply a gate time period (Td). Then, the probability analyzer receives the gate time period, compares a mean transition count of the mean frequency of transitions to a transition probability (P), and supplies a compensated transition count (Nc) as follows:
Nc=Np/P.
The probability analyzer generates a mean transition count (Np) of frequency transitions over the time period Td and derives a mean data stream rate (B) as follows:
B=(Np)/(Td)(P).
In a system for selecting the frequency range of a VCO for use in pseudorandom NRZ communications, the system further comprises a decoder to accept the compensated transition count, determine a frequency range corresponding to the compensated transition count, and supply a frequency range selection command at an output. The system further comprises a multiband voltage controlled oscillator having an input to accept a tuning voltage, an input to accept the frequency range selection command, and an output to supply a voltage controlled oscillator frequency responsive to the tuning voltage and frequency range selection.
Additional details of the above-described system, and a method for measuring a mean NRZ data stream rate are presented below in greater detail.